This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2000-111520 filed on Apr. 13, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a common rail fuel injection system in which highly pressurized fuel accumulated in a pressure accumulation chamber of a common rail housing is supplied to injectors of an internal combustion engine, and in particular to the construction of the common rail housing.
2. Description of Related Art
A common rail fuel injection system as disclosed in JP-A-4-287866 is well known. The injection system has a common rail housing which acts as a kind of a surge tank and is provided with a pressure accumulation chamber extending in a lateral direction thereof. Highly pressurized fuel accumulated in the pressure accumulation chamber is supplied to injectors of an internal combustion engine.
In the conventional common rail system, as shown in FIG. 6, a common rail housing 101 is provided with a pressure accumulation chamber 102 in which highly pressurized fuel is temporally accumulated, a plurality of fuel supply ports (not shown) through which highly pressurized fuel delivered by a fuel supply pump is supplied to the pressure chamber 102, and a plurality of fuel delivery ports 103 through which the highly pressurized fuel in the pressure accumulation chamber 102 is delivered to injectors installed in respective cylinders of the internal combustion engine.
A cylindrical pressure accumulation pipe 104, whose outer circumference is formed in round shape, constitutes the pressure chamber 102. A cylindrical outlet pipe 105, which is connected to the cylindrical pressure accumulation pipe 104, constitutes each of the fuel delivery ports 103. The cylindrical pipe 105 is provided at an outer circumferential surface on a leading end thereof with a male thread portion 106 into which a high-pressure pipe is screwed for fastening.
In the common rail housing 101 of the conventional fuel injection system, the pressure chamber 102 is formed in complete round shape in cross section so that tensile stresses are induced at and concentrated on intersecting portions 107 where the pressure chamber 102 and the respective fuel delivery ports 103 intersect each other. As the fuel injection pressure in the system is higher, the stresses induced at and concentrated on the intersecting portions 107 are larger so that reliability in strength of the common rail housing 101 is likely to be jeopardized. Accordingly, the conventional common rail housing 101 has a drawback on realizing the fuel injection system with much higher pressure.
As a way of solving the drawback, it may be contemplated to reduce an inner diameter of the pressure chamber 102 so that the stresses induced at the intersecting portions 107 become smaller. However, as it is necessary to secure a given inner volume of the pressure chamber 102, which is predetermined for one or more injectors, for a purpose of limiting injection fluctuation of injectors, a longitudinal length of the common rail housing 101 is obliged to be longer so that manufacturing efficiency of the common rail housing 101 and installation efficiency thereof to the internal combustion engine are adversely affected.
As another way of reducing the stresses, JP-A-10-169527 discloses, as shown in FIGS. 7A to 7C, common rail housing 101 having an oval shaped pipe 114 provided with an oval shaped pressure accumulation chamber 113 and also provided with a fuel supply or delivery bore 115 connected to the pressure accumulation chamber 111 at a position where a curvature of the oval is small.
It also describes that the oval shaped pipe 114 having the oval shaped pressure accumulation chamber 113 as shown in FIG. 7B is shaped by plastically deforming a complete round pipe 112 having a pressure accumulation chamber 111 as shown in FIG. 7A with press working or roll forming. In the deformation process, residual stresses (tensile stresses) are induced in the common rail housing 101. Accordingly, stresses (xcex1) always remain at the intersecting portion 116 where the fuel input or output bore 115 is connected to the pressure accumulation bore as shown in FIGS. 7B and 7C. Even if the tensile stresses at the intersecting portion, which is induced by inner pressure (P) of highly pressurized fuel in the pressure accumulation chamber 113, is reduced, a sum of the residual stresses (xcex1) and the stresses due to the inner pressure (P) is likely to be relatively large so that strength at the intersecting portion 116 is jeopardized.
If wall thickness of the oval shaped pipe 114 is equal to or more than a diameter of the oval pressure accumulation chamber 113 in a minor axis of the oval thereof, that is, if an outer diameter (for example, 30 mm) of the oval shaped pipe 114 in a minor axis length of the oval thereof is larger by three times or more than the diameter (for example, 10 mm) of the oval pressure accumulation chamber 113 in a minor axis of the oval thereof, the strength at the intersecting portion 116 can be sufficiently assured.
However, deformation of the complete round pipe made of iron steel into the oval shaped pipe 114 having such a thick thickness wall needs a several tens or hundreds pressing load. Even if deformed by the press working or the roll forming, as shown in FIGS. 8A and 8B, the oval shaped pipe 114 is likely to have wrinkles or cracks so that pressure endurance of the oval shaped pipe 114 is reduced.
An object of the invention is to provide a common rail fuel injection system having a common rail housing in which stresses concentrated on given portions are remarkably reduced and, further, resisting pressure strength is distinctively improved.
It is another aspect of the invention to provide a method of manufacturing the common rail housing in which residual stresses induced therein are limited.
To achieve the above objects, the common rail housing is composed of a pressure accumulation pipe, whose outer circumference is formed in roughly complete round shape in cross section, being provided inside with a pressure accumulation chamber having given shaped cross section and extending in a longitudinal direction thereof, and a branch pipe being provided inside with a fuel conduit bore. Outer circumference of the given cross section has a first portion whose curvature is smaller than that of complete round shape having an area equal to that of the given cross section. The branch pipe is connected to the pressure accumulation pipe so that the fuel conduit bore and the pressure accumulation chamber intersect nearly perpendicularly to each other at the first portion.
It is preferable that wall thickness of the pressure accumulation pipe at the first portion is thickest. Therefore, when highly pressurized fuel is supplied to the pressure accumulation chamber, not only the stresses concentrated on the first portion are limited but also wall strength of the pressure accumulation pipe at the first portion is reinforced.
Accordingly, the common rail housing may be made of relatively low hardness material such as low carbon steel that is easily formed in the given shape on manufacturing. Further, as the wall thickness of the pressure accumulation pipe is relatively thick at the first portion and relatively thin at the portions other than the first portion around the circumference of the pressure accumulation chamber, the wall thickness of the pressure accumulation pipe of the present invention is thinner as a whole than the conventional common rail housing, resulting in saving fuel consumption.
It is preferable that the given shaped cross section of the pressure accumulation chamber is formed in roughly flat oval shape or in roughly oval shape.
To limit residual stresses to be induced on manufacturing the common rail housing mentioned above, it is preferable that, after forming a product whose outside configuration is equal to that of the pressure accumulation pipe, the product is drilled at first in a longitudinal direction thereof to form a round hole whose diameter is equal to a minor axis length of the oval in cross section of the pressure accumulation chamber and, then, opposite sides of an inner wall of the round hole are removed in a longitudinal direction thereof by broaching or electrical discharge machining. With the processes, the pressure accumulation chamber having the oval shaped cross section is formed.